1. Field of the Invention
The present invention relates to a planar radiation detector which uses a radiation-induced-charge conversion film or a light-induced-charge conversion film formed of amorphous selenium.
2. Description of the Related Art
Conventionally, the planar X-ray detector is known to be used in medical X-ray radiography for reducing the exposure dose of a subject and improving diagnostic performance. In the planar X-ray detector, an X-ray-induced-charge conversion film, which is sensitive to X rays, is used, and a distribution of electric charge produced in the X-ray-induced-charge conversion film by X rays is read by use of light or a great number of electrodes for recording the distribution of electric charge. Specifically, the X-ray-induced-charge conversion film is arranged in the planar radiation detector. When the X-ray-induced-charge conversion film is irradiated with X rays, the distribution of electric charge is produced in correspondence with the energy of the X rays, and is then read out in the form of an electric signal. The X-ray-induced-charge conversion film is conventionally made of amorphous selenium (a-Se).
However, in the case where the X-ray-induced-charge conversion film of amorphous selenium is used in a static-image type planar radiation detector such as a mammography system or another radiography system, electrons are trapped by electron traps and a spatial distribution of negative charge is produced in the vicinity of a so-called through region of the X-ray-induced-charge conversion film (i.e., a region of the X-ray-induced-charge conversion film exposed to an extremely high dose of X rays, which is higher than the normal dose by two orders of magnitude), so that a phenomenon called a ghost, in which the sensitivity is lowered, occurs. The ghost is particularly conspicuous in the case where the upper electrode, through which the X rays enter the X-ray-induced-charge conversion film, is negatively biased for protection of TFTs and the like.
Incidentally, the planar radiation detector is a detector which takes radiographic images at intervals of at least 0.3 seconds, and the intensity of X rays incident on the planar radiation detector is normally an order of magnitude higher than the intensity of X rays incident on the moving-image type detector which takes radiographic images at intervals of at least 0.03 seconds.
In addition, another type of planar radiation detector in which a fluorophor converts X rays into visible light, and a thin selenium film is used as a light-induced-charge conversion film is also known. In this type of planar radiation detector, the negative bias is also used for protection of TFTs and the like. Therefore, in this case, the ghost also becomes conspicuous since electrons also mainly move in the light-induced-charge conversion film.
As explained above, the performance of the conventional X-ray-induced-charge conversion film and light-induced-charge conversion film against the ghost is insufficient in the case where the X-ray-induced-charge conversion film and light-induced-charge conversion film are used for taking static images, in particular, when the upper electrode is negatively biased.
For example, Japanese Unexamined Patent Publication No. 2003-229596 discloses an X-ray photoelectric converter in which the halogen concentration in an X-ray-induced-charge conversion film is suppressed to or below 1 ppm for enhancing responsiveness in dynamic X-ray radiography (using concurrent movement of carriers of both polarities) in consideration that halogen causes electron trapping in the X-ray-induced-charge conversion film. However, even when only the halogen concentration is limited, the phenomenon of lowering the sensitivity can still occur according to the lots of raw material.
In addition, U.S. Pat. Nos. 5,880,472 and 6,171,643 discloses an X-ray-induced-charge conversion film made of amorphous selenium doped with As and Cl. In this X-ray-induced-charge conversion film, the phenomenon of lowering the sensitivity can also occur.
Further, U.S. Patent Application Publication No. 20030223534 discloses reduction of electron trapping in a film by doping the film with alkali metal for enhancing responsiveness in dynamic X-ray radiography (using concurrent movement of carriers of both polarities) as Japanese Unexamined Patent Publication No. 2003-229596. However, even when the film is doped with alkali metal, the phenomenon of lowering the sensitivity can still occur according to the lots of raw material.
The present inventors consider that the phenomenon of lowering the sensitivity is greatly related to concurrent movement of both of electrons and positive holes in a film.